Large-area, free-standing metal oxide films and transistors made therefrom

ABSTRACT

The present invention provides continuous, free-standing metal oxide films and methods for making said films. The methods are able to produce large-area, flexible, thin films having one or more continuous, single-crystalline metal oxide domains. The methods include the steps of forming a surfactant monolayer at the surface of an aqueous solution, wherein the headgroups of the surfactant molecules provide a metal oxide film growth template. When metal ions in the aqueous solution are exposed to the metal oxide film growth template in the presence of hydroxide ions under suitable conditions, a continuous, free-standing metal oxide film can be grown from the film growth template downward into the aqueous solution.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with United States government support awarded bythe following agencies: the National Science Foundation (NSF), undergrant number DMR-0905914 and the Air Force Office of Scientific Research(AFOSR) under grant number FA9550-091-0482. The United States governmenthas certain rights in this invention.

BACKGROUND

Zinc oxide is a promising semiconductive material due to its largebandgap and binding energy. In addition zinc is an abundant andrelatively expensive alternative to silicon. However, there aresignificant challenges to making zinc oxide thin films. For example, thetechniques used for silicon thin film manufacturing generally cannot beapplied to the manufacture of metal oxide films. Although many metaloxide films, including zinc oxide films, can be grown on a solidsubstrate, there are no known chemical etchants that are able toselectively remove the underlying substrate and allow for the lift-offof the metal oxide film. In addition, although films of polycrystallinemetal oxides, such as zinc oxide, can be formed using processes such assputter deposition and chemical vapor deposition, these polycrystallinefilms do not have the desirable semiconducting properties that certainsingle-crystalline metal oxide films can provide.

SUMMARY

One aspect of the present invention provides a continuous, free-standingmetal oxide film comprising at least one continuous, single-crystallinemetal oxide domain having an area of at least 0.1 μm². In someembodiments the continuous, free-standing metal oxide films have athickness of no greater than 10 μm. In some embodiments, the at leastone continuous, single-crystalline metal oxide domain has an area of atleast 1 μm². In still other embodiments, the at least one continuous,single-crystalline metal oxide domain has an area of at least 100 μm².The metal oxide film can be a flexible large-area film, having an areaof at least 1 cm² and a thickness of 1000 nm or less.

Another aspect of the invention provides methods of making metal oxidefilms. In one embodiment the method comprises forming a surfactantmonolayer at the surface of an aqueous solution, wherein the headgroupsof the surfactant molecules provide a metal oxide film growth template,and exposing metal ions in the aqueous solution to the metal oxide filmgrowth template in the presence of hydroxide ions under conditions thatpromote the growth of a metal oxide film on the metal oxide film growthtemplate. Sodium dodecyl sulfate (SDS) is an example of a surfactantthat can be used to form the metal oxide film growth template.

Continuous, free-standing zinc oxide films are one example of the typeof thin films that are provided by the present invention. In someembodiments of the zinc oxide films, the zinc oxide comprises thewurtzite phase of zinc oxide. In other embodiments, the zinc oxidecomprises a Zn_(0.75)O_(x) structure. Other continuous, free-standingmetal oxide films that can be produced using the present methods includemagnesium oxide films, copper oxide films, titanium oxide films, tinoxide films and barium titanium oxide films.

Another aspect of the invention provides transistors incorporating thepresent metal oxide films as a semiconductor active layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the molecular structure of sodium dodecyl sulfate (SDS).

FIG. 2 is an illustration of the spatial distribution of dodecylsulfates, zinc cations and hydroxide anions in a reaction container.

FIG. 3 shows the proposed interaction between dodecyl sulfate headgroupsand a zinc oxide (0001) surface.

FIG. 4 shows the proposed mechanism for the formation of a large areazinc oxide single crystal film.

FIG. 5 shows a free standing ZnO film. A. An optical microscope image ofa ZnO film supported on a silicon substrate. B. A TEM image of a ZnOfilm; the inset is the diffraction pattern taken from the film.

FIG. 6 shows: (A) an optical microscopy image of the zinc oxide squarecrystals; (B) the transmission electron microscopy image of the zincoxide square crystal, as well as (C) its electron diffractions pattern;and (D) an illustration of the proposed mechanism for binding dodecylsulfates with the (0001) surface of a zinc oxide square crystal.

FIG. 7 illustrates a method for making a thin film transistor from amultilayered metal oxide film structure.

FIG. 8 is a diagram of a bottom-gated thin film transistor incorporatinga semiconducting metal oxide film as an active layer.

FIG. 9( a) shows the drain current as a function of drain voltage atvarious gate voltages for a thin film transistor incorporating a ZnOfilm. FIG. 9( b) shows the on/off ratio for the thin film transistor.FIG. 9( c) shows a comparison of the on/off ratio for the thin filmtransistor made with an annealed ZnO film to the on/off ratio for a thinfilm transistor without an anneal step

DETAILED DESCRIPTION

The present invention provides continuous, free-standing metal oxidefilms and methods for making and processing said films. The methods areable to produce large-area, flexible, thin films having one or morecontinuous, single-crystalline metal oxide domains. The flexible metaloxide films can be easily transferred from the surface of the liquid onwhich they are grown to a variety of substrates, including flexibleplastic substrates. In addition, the metal oxide films can be made witha large enough area to be patterned lithographically.

Various embodiments of the metal oxide films can be transparent and/ornon-toxic and can have a wide bandgap, high mobility, and/orpiezoelectric properties. As such, the present metal oxide films can beused in flexible electronics, including thin film transistors (TFTs),solar cells, light-emitting diodes for the ultraviolet (UV) spectrum,laser diode PN-junctions, and electrodes.

As used herein, the phrase “continuous, free-standing film” refers to afilm that is continuous in three dimensions and has a self-supportingstructure that does not include an organic binder or matrix and does notinclude an underlying support substrate that provides it with itsself-supporting structural integrity. The “continuous, free-standingfilms” have a substantially uniform composition in that they are notcomposed of or from an array or collection of a plurality of discretenanoparticles, such as nanoparticles, nanowires or nanorods.

In a basic embodiment, the present methods include the steps of forminga surfactant monolayer at the surface of an aqueous solution, whereinthe headgroups of the surfactant molecules in the monolayer packtogether to provide a soft, organic metal oxide film growth template.When metal ions in the aqueous solution are exposed to the film growthtemplate in the presence of hydroxide ions under suitable conditions, ametal oxide film can be grown from the film growth template in alayer-by-layer fashion downward into the aqueous solution. Thus, thefilms can be characterized by a thickness dimension extending downwardfrom the film growth template and into the aqueous solution, as well asby length and width dimensions which extend laterally through the film,perpendicular to the thickness dimension.

As illustrated in the Example, below, the metal oxide films can be grownfrom a plurality of nucleation sites on the film growth template.Because the small single-crystalline metal oxide domains growing at eachnucleation site are able to rotate with respect to one another, multiplesmaller domains are able to converge into larger, single-crystallinedomains within the film. Thus, some embodiments of the present methodsare able to produce continuous metal oxide films having at least onecontinuous, single-crystalline metal oxide domain with an area of atleast 0.1 μm², where the area is determined based on the length andwidth dimensions of the film. This includes embodiments of the methodsthat produce continuous metal oxide films having at least onecontinuous, single-crystalline metal oxide domain with an area of atleast 1 μm², at least 10 μm², at least 100 μm², or even at least 1000μm². As illustrated in the Example, below, the single-crystallinedomains can be in the form of different phases, depending upon thereactions conditions and the duration of the film growth.

Although the present metal oxide films are exemplified by zinc oxidefilms in the Example, below, a variety of metal oxide films can beproduced using the present methods. These include metal oxide films thatcannot be produced using conventional solution phase growth processes.Examples of other metal oxide films that can be grown using the presentmethods include magnesium oxide films, copper oxide films, tin oxidefilms, titanium oxide films and barium titanate films.

The surfactants that form the metal oxide film growth template should bechosen such that the charge-bearing headgroup atoms in the surfactantmonolayer are sized and spaced to promote the growth of the metal oxideof interest. Thus, the distance between the metal atoms in the growingmetal oxide film should be no less than the distance between thecharge-bearing atoms of the surfactant headgroups. For example, in someembodiments, the distance between charge-bearing atoms of the surfactantheadgroups is about 20 to 30% greater than the distance between themetal atoms of the metal oxide. Sodium dodecyl sulfate is an example ofa surfactant having a sulfate headgroup that can be used to form themetal oxide film growth template for the growth of a variety ofcontinuous, free-standing metal oxide films.

The dimensions of the present metal oxide films depend on a variety offactors, including the dimensions of the container in which the filmsare grown, the reaction conditions and the duration of the film growthprocess. For example, the film growth conditions can be designed to growa metal oxide thin film having an area that is limited only by the areaof the air-aqueous solution interface in the container in which thereaction is carried out. In some embodiments, the metal oxide film has asurface area of at least 1 mm². This includes embodiments in which themetal oxide film has a surface area of at least 1 cm², at least 10 cm²and at least 100 cm².

The thickness of the films depends, at least in part, on theconcentration of reactants (e.g., a source of hydroxide ions and metalions) and the temperature and duration of the film growth reaction. Ifthe films are to be used in flexible electronics, they are desirablysufficiently thin to provide a flexible film. For example, in someembodiments, continuous, free-standing metal oxide films having athickness of up to 10 μm are grown. This includes metal oxide filmshaving a thickness of up to 5 μm, up to 1 μm, up to 500 nm, up to 100nm, up to 50 nm, and up to 20 nm. For example, in accordance withvarious embodiments of this invention, metal oxide films havingthicknesses in the range from 10 to 500 nm, from 10 to 100 nm, and from10 to 50 nm can be grown. An example of an electronic device into whichthe present metal oxide films can be incorporated is a thin filmtransistor, in which metal oxide thin films can provide transparentelectrodes.

The as-grown films can be in the form of a multilayered structurecomprising a stack of metal oxide films. In order to prepare suchmultilayered metal oxide film structures for use in devices, such asTFTs, the thin film layers should be separated. FIG. 7 illustrates amethod for separating the metal oxide thin films in a multilayeredstructure and forming a TFT from a single metal oxide film. In thismethod a multilayered metal oxide film structure 700 is disposed on asubstrate 702. A second, adhesive substrate 704 is prepared. Theadhesive substrate can include an adhesive layer 706 (e.g., SU-8) and aflexible polymeric layer 708 (e.g., polyethylene teraphthlate). Theadhesive substrate 704 is contacted with one or more of the top metaloxide film layers of the multilayered structure 710. When the adhesivesubstrate is lifted away from the multilayered structure, the top metaloxide film layers 710 are separated. This “adhere, lift and separate”process can be repeated multiple times until a single layer of metaloxide film 712 remains on an adhesive structure. In the example of FIG.8, the process is repeated with a second adhesive substrate 714 and athird adhesive substrate 716. A TFT can be fabricated from the resultingstructure by depositing a source electrode 718, a drain electrode 720, agate dielectric 722 and a gate electrode 724 onto the metal oxide activelayer 712.

An example of a bottom-gated TFT is shown in FIG. 8. This TFT includes ametal oxide film 800 as an active layer, a source electrode 802, a metalcontact 804 for the source electrode, a drain electrode 806, a metalcontact 808 for the drain electrode, a gate dielectric 810, a gateelectrode 812 and an insulating layers 814, 816.

Example

This example describes the formation of continuous single-crystallizingzinc oxide films in accordance with the present invention.

Synthesis:

A reaction vial containing 25 millimolar (mM) zinc nitrate, 25 mMhexamethylenetetramine (HMTA), and 10 mM sodium dodecyl sulfate (SDS)solution was placed in a convection oven set at 60° C. After thereaction was conducted for 5 hours, a continuous visible zinc oxide filmformed on the surface of the reaction solution, covering the entirewater-air interface. The film is free-standing and can be transferredonto various substrates just by immersing a substrate in the solutionand carefully lifting the film with the substrate.

Although the reaction of this example is desirably conducted at atemperature of at least 60° C., any temperature from about 60° C. to 90°C. can also be employed. Concentrations other than 25 mM for zincnitrate and HMTA can also be employed. For example, the reaction can becarried out at zinc nitrate and HMTA concentrations from 10 mM to 100mM. The concentrations of these two chemicals are related to each other.The molar ratio of HMTA to zinc nitrate is usually 1:1, but can also be,for example, up to 2:1 (that is, the concentration of HMTA can belarger, but should not be smaller than that of zinc nitrate). Theconcentration of SDS can be in the range from 5 mM to 40 mM, forexample. This concentration can be independent from the concentrationsof the other two chemicals.

A continuous zinc oxide film covering the entire surface of the aqueoussolution can be achieved within one hour after the reaction starts.Longer reaction times can be used to obtain larger single crystaldomains within the metal oxide film.

The reaction container in which the metal oxide film is grown can haveany suitable shape and size, and can be made of a variety of materials.Suitable materials for the reaction container include glass, plastic,PTFE and the like.

Growth Mechanism:

Zinc nitrate is used in this example to provide zinc cations when it isdissolved in water. HTMA is an organic base that hydrolyzes in water andgenerates ammonia and formaldehyde at a slow rate. Ammonia is a weakbase in water and hydrolyzes automatically to release hydroxide anions.Zinc cations and hydroxide anions react and generate zinc hydroxide,which is further dehydrated to provide zinc oxide. The chemicalreactions are as follows:

SDS is a widely used anionic surfactant with a 12-carbon hydrocarbonchain, a sulfate anion headgroup, and a sodium cation counter ion. FIG.1 shows a detailed molecular structure of SDS, which includes ahydropholic hydrocarbon chain 102 and a hydrophilic head group 104. Whendissolved in water, SDS is ionized. The sodium cations are uniformlydissolved while the amphiphilic dodecyl sulfate anions are absorbed atthe surface of the solution and form an electrical negative layer withthe hydrophobic surfactant chains pointing up (away from the solution)and the anionic headgroups pointing down (buried in the solution). Avery large amount of SDS (e.g., a concentration near or larger than thecritical micelle concentration of, about 9 mM) was added to the reactionsystem to ensure a saturated surface absorption and tight packing of thesurfactant molecules. As a result, the dodecyl sulfate groups form anegatively charged molecular monolayer at the liquid solution-airinterface.

This negatively charged molecular monolayer serves as a two-dimensionalsoft planar template for the growth of the metal oxide film. This planartemplate has strong interactions with both zinc cations and the surfaceof the growing zinc oxide film. Since each divalent zinc cation carriestwo positive charges, they are concentrated near the negatively chargedtemplate due to coulombic forces. The concentrated zinc cations alsoaccelerate hydroxide anion diffusion to the surface of the aqueoussolution where they react with the zinc cations and generate zinc oxideat the template. The spatial distribution of dodecyl sulfates 202, zinccations 204, and hydroxide anions 206 in the reaction solution 208 areillustrated in FIG. 2.

The planar metal oxide growth template formed by the surfactantmolecules also has strong affinity with the (0001) surface of zincoxide. Each dodecyl sulfate group has a −1 charge, which is equallydistributed to its three oxygen atoms that are not bonded to a carbonatom. On the other hand, the (0001) surface of zinc oxide is composed ofonly zinc cations and thus is positively charged. Therefore, there iscoulombic force between the template and zinc oxide surface, inparticular, the (0001) surface of the zinc oxide nuclei formedunderneath the template. Moreover, the partially negatively chargedoxygen atoms of the dodecyl sulfate molecules can act like the (000-1)surface of zinc oxide, so it is very likely that these oxygen atoms bondwith zinc cations on the (0001) surface in the same way that the oxygenanions and zinc cations do in a zinc oxide crystal. Due to this atomicconfiguration, these oxygen atoms can provide a transition area betweenthe zinc oxide film and the dodecyl sulfate template. A proposedmechanism for the interaction between a dodecyl sulfate group and thezinc oxide (0001) surface of a growing zinc oxide film is illustrated inFIG. 3.

The present methods employ a soft organic metal oxide growth templatethat allows nucleation sites to rotate such that the single-crystallinemetal oxide domains growing at neighboring nucleation sites are able toconverge into a single, continuous, single-crystalline domain.Specifically, the smaller nucleation sites can rotate around theirc-axis until they adopt an orientation that matches that of the othernucleation sites. As the smaller single-crystalline domains merge into alarger single-crystalline domain, the supply of zinc cations andhydroxide anions help to “sew together” the edges. FIG. 4 illustratesthe formation mechanism of a ZnO film.

As shown in FIG. 4, the surfactant headgroups 402, shown here as SDShead groups, self-organize into a growth template 404 on the surface ofthe growth solution. The oxygen atoms in the head group then bind withZn atoms 406 from the solution to provide a growing single-crystallinedomain 408. Bonding 407 between growing domains 408, 410 can occur andthe resulting bonds can pull the smaller domains together to form asingle, larger, single-crystalline domain 412.

It is commonly believed that the (0001) surface grows faster than anyother surfaces of zinc oxide and the (000-1) surface is an inertsurface, that is, it grows slowest. In the present methods, the (0001)surface is associated with the dodecyl sulfate groups of the surfactantmolecules and faces toward the water surface, while the inert (000-1)surface faces down toward the aqueous bulk phase. Therefore, the zincoxide grows laterally and forms in the morphology of a thin film.

An image of a zinc oxide film made in accordance with the presentmethods is shown in FIG. 5A. The single-crystalline domain can reach anarea of hundreds of square micrometers, or greater. The thickness of theZnO film can range, for example, from tens to hundreds of nanometersdepending on the growth time and temperature. Most of thesingle-crystalline pieces still exhibit a hexagonal shape as shown inFIG. 5B. The corresponding electron diffraction pattern revealed ac-plane orientation (inset of FIG. 5B).

Formation of Zinc Oxide Square Crystals:

When the reaction time was extended to more than 10 hours, themorphology of the single crystal zinc oxide domain changed fromhexagonal crystal to square crystal. This is shown in FIG. 6A. Thecrystal structure also changed from the common Wutzite structure(hexagonal) to a zinc vacancy-induced rectangular structure. For thepurposes of this disclosure, this structure is referred to as the“square phase”. The corresponding TEM image and electron diffractionpattern are shown in FIGS. 6B and C, respectively. This square phase isalso known as the Zn_(0.75)O_(x) structure, and is described in Ding etal, Solid State Communications, Vol. 138, pp. 390-394 (2006).

The transition from hexagonal crystal to this square phase is believedto be due to the surface change imbalance of the hexagonal crystal. Aswas discussed previously, each dodecyl sulfate anion has a charge of −1,which is equally distributed to the three oxygen atoms that bind withthe zinc cations on the zinc oxide (0001) surface. That is, each oxygenatom has −⅓ charge. On the other hand, each zinc cation on the zincoxide (0001) surface has a positive one half charge. Therefore, thesurface charge of the zinc oxide (0001) surface is not fully neutralizedby dodecyl sulfate anions. The surface charge imbalance is 33%. Given along reaction time, both the dodecyl sulfate anions and zinc oxidecrystal can reorganize themselves to eliminate the charge imbalance.Eventually, one of the three oxygen-zinc bonds formed between thedodecyl sulfate group and zinc oxide crystal will break. Thus, the −1charge of the dodecyl sulfate anion is now shared by two oxygen anions,which are bonded with two zinc cations. Under this situation, the chargebetween the SDS and zinc oxide is completely balanced and the entiresystem reaches a more stable state.

Such reorganization will generate a significant number of zinc vacanciesin the zinc oxide crystal. Considering the crystal structure of zincoxide, one stable phase for zinc oxide having a large number of zincvacancies is a phase in which the zinc oxide crystal loses one quarterof its zinc cations (left of FIG. 6D). Since the number of dodecylsulfate anions that are absorbed on the zinc oxide (0001) surface inunit area remains the same, the surface charge imbalance can be reducedto 11% through this reorganization. As illustrated in FIG. 6D, thisconfiguration creates two types of zones on the zinc oxide (0001)surface. In Zone I, each dodecyl sulfate group is bonded with two zinccations. The charge between the surfactant and the crystal is completelybalanced in this region. This region contains ¾ of the total SDSmolecules. Another one quarter of the SDS molecules are located in zoneII, where they bond with three zinc cations. Therefore, the charge inthis region is still unbalanced. Nonetheless, stabilization of the zincoxide crystal and the interface charge together make this square phasedzinc oxide thin film into a stable structure.

The as-grown metal oxide films may take the form of a multilayeredstructure comprising a stack of two of more metal oxide films. If thisis the case, the multilayered structures can be further processed torender them suitable for use in electronic applications, such as TFTs.This further processing includes lifting the multilayered structure fromthe growth solution with a substrate and annealing the structure todrive out moisture and passivate the film surfaces. For example the ZnOfilms can be annealed at 300° C. for 100 minutes. A single ZnO film canthen be separated onto an adhesive substrate using the methodillustrated in FIG. 7. The performance characteristics for abottom-gated TFT made using these methods are shown in FIG. 9. Thematerials in this TFT are a ZnO active, layer, a SiO₂ gate dielectric, aheavily doped Si gate electrode, and SiO₂ and SiN_(x) insulating layers.FIG. 9( a) shows the drain current as a function of drain voltage atvarious gate voltages. FIG. 9( b) shows the on/off ratio for the device.FIG. 9( c) shows a comparison of the on/off ratio for the TFT made withthe annealed ZnO film to the on/off ratio for a TFT without an annealstep.

As used herein, and unless otherwise specified, “a” or “an” means “oneor more.” All patents, applications, references, and publications citedherein are incorporated by reference in their entirety to the sameextent as if they were individually incorporated by reference.

As will be understood by one skilled in the art, for any and allpurposes, particularly in terms of providing a written description, allranges disclosed herein also encompass any and all possible subrangesand combinations of subranges thereof. Any listed range can be easilyrecognized as sufficiently describing and enabling the same range beingbroken down into at least equal halves, thirds, quarters, fifths,tenths, etc. As a non-limiting example, each range discussed herein canbe readily broken down into a lower third, middle third and upper third,etc. As will also be understood by one skilled in the art, all languagesuch as “up to,” “at least,” “greater than,” “less than,” and the likeincludes the number recited and refers to ranges which can besubsequently broken down into subranges as discussed above. Finally, aswill be understood by one skilled in the art, a range includes eachindividual member.

It is specifically intended that the present invention not be limited tothe embodiments and illustrations contained herein, but include modifiedforms of those embodiments including portions of the embodiments andcombinations of elements of different embodiments as come within thescope of the following claims.

1. A continuous, free-standing metal oxide film having a thickness of nogreater than 10 μm and comprising at least one continuous,single-crystalline metal oxide domain having an area of at least 0.1μm².
 2. The metal oxide film of claim 1, wherein the at least onecontinuous, single-crystalline metal oxide domain has an area of atleast 1 μm².
 3. The metal oxide film of claim 1, wherein the at leastone continuous, single-crystalline metal oxide domain has an area of atleast 100 μm².
 4. The metal oxide film of claim 1, having an area of atleast 1 cm².
 5. The metal oxide film of claim 1, wherein the metal oxideis zinc oxide.
 6. The metal oxide film of claim 5 having a thickness nogreater than about 100 nm.
 7. The metal oxide film of claim 5, whereinthe zinc oxide comprises the wurtzite phase of zinc oxide.
 8. The metaloxide film of claim 5, wherein the zinc oxide comprises a Zn_(0.75)O_(x)structure.
 9. The metal oxide film of claim 1, further comprising asurfactant associated with the surface of the metal oxide.
 10. The metaloxide film of claim 1, wherein the metal oxide is at least one of amagnesium oxide, a copper oxide, a titanium oxide, a tin oxide or abarium titanium oxide.
 11. A continuous, free-standing zinc oxide filmcomprising a Zn_(0.75)O_(x) structure.
 12. The zinc oxide film of claim11, comprising at least one continuous, single-crystallineZn_(0.75)O_(x) domain having an area of at least 0.1 μm².
 13. The zincoxide film of claim 11, comprising at least one continuous,single-crystalline Zn_(0.75)O_(x) domain having an area of at least 1μm².
 14. The zinc oxide film of claim 11 having a thickness of nogreater than about 1000 nm.
 15. A method of making a metal oxide film,the method comprising: (a) forming a surfactant monolayer at the surfaceof an aqueous solution, wherein the headgroups of the surfactantmolecules provide a metal oxide film growth template; and (b) exposingmetal ions to the metal oxide film growth template in the presence ofhydroxide ions under conditions that promote the growth of a metal oxidefilm at the metal oxide film growth template.
 16. The method of claim15, wherein the metal ions are zinc ions.
 17. The method of claim 16,wherein the surfactant molecules are dodecyl sulfate molecules.
 18. Themethod of claim 16, wherein the metal oxide film comprises aZn_(0.75)O_(x) structure.
 19. The method of claim 15, wherein the metaloxide film is a continuous, free-standing metal oxide film and comprisesat least one continuous, single-crystalline metal oxide domain having anarea of at least 0.1 μm².
 20. A transistor incorporating the metal oxidefilm of claim 1 as an active layer.